In denying an appeal by a conservative group called the Wisconsin Club for Growth, the justices left intact a federal appeals court ruling from last September that overturned an earlier federal district court decision that had halted the investigation.

Walker, a rising star in the Republican Party, is serving his second term as Wisconsin's governor after winning re-election in November 2014.

The Wisconsin Supreme Court is currently considering separate cases concerning whether the investigation should continue. Attorneys representing the state-appointed investigators in the case noted in court filings that the investigation is dormant while the state court determines whether the alleged actions violated Wisconsin campaign finance laws.

The Wisconsin Club for Growth wanted the Supreme Court to throw out the appeals court ruling. The focus of the probe is on possible illegal coordination between Walker's campaign and conservative special interest groups in 2011 and 2012.

A federal judge in May 2014 stopped the probe after the Wisconsin Club for Growth filed a lawsuit accusing investigators of sidelining it from political activities and violating its rights to free speech, association and equal protection. Walker was elected governor in 2010.

In June 2012, he became the first governor in U.S. history to survive a recall election. The investigation was launched two months later and is also looking at recall elections in 2011 involving other candidates.

The case is O'Keefe v. Chisholm, U.S. Supreme Court, No. 14-872. (Reporting by Lawrence Hurley; Editing by Will Dunham)